Floor board strength is probably a major point for those rails. And a stiffer floor structure would impact the whole chassis stiffness.

I cut the rear seat mount braces out on my XP car. I was amazed at how much the passenger floor flexes when I step on it now. I left the front seat mounting point as I think that would make the floor way too flimsy. And this is with the FM frame rails. I rarely have a passenger seat installed but it is a nice option. Plus I end up in that area whenever I have to play with the ECU wires.

I love these I have been thinking about doing this as my stock frame rails do not like having my trans cross member welded to them. Now that I have seen some pics of the floor and rails cut open I will be doing this in the near future!!!!

But at the stage im at in the build, its time to consider frame rails of sorts. Although this looks like a very solid solution, would there be any benefit to virtually re creating the FM style frame rails but with thicket steel to reinforce the oem rail section vs cutting it all out and placing 2" tube steel in there as OP has done? Seems like doing it this way would yield almost the same result with less effort.